Electronic circuits and the electronic components mounted thereon have become increasingly more compact. As a result, attempts have been made to utilize the printed circuit boards themselves as a means for dissipating heat. Composite printed circuit boards typically contain a metal core layer made of copper, steel or aluminum and an insulating layer made of plastic or enamel overlying the metal core layer. It has been previously suggested to use the metal core layer to dissipate heat.
While the metallic core layer of existing composite printed circuit boards may afford some thermal conductivity, the insulating layer used in conjunction with such metal cores has not been entirely satisfactory. For example, plastic insulating layers have poor temperature resistance. Moreover, since the plastic layer must also be relatively thick, because of the manufacturing techniques used to apply plastics, a high thermal resistance can occur between the conducting layer and the metallic core which can sharply reduce heat dissipation in the core layer. In addition, the elevated dielectric constant of conventional plastic materials leads in some applications to unacceptable capacitance between the individual printed circuits and/or between the conducting layer and the core layer.
Enamel insulating layers have also been suggested. Those materials may resolve some of the problem of temperature sensitivity, and may afford better heat conductivity than plastics, but enamel layers are difficult to apply, particularly in thin layers, and are too porous. Coating inaccessible spots, such as corners, or inner surfaces of boreholes, is not practical using enamels. Composite printed circuit boards typically contain boreholes to connect different layers of the printed circuit, or to connect electronic components mounted on the boards.
Heat generation of electronic components can present additional problems. Due to the different coefficients of expansion of the printed circuit (and the composite layers therein) thermal stresses can arise between the layers. For this reason--particularly ih the presence of repeated mechanical stresses--solder joints connecting the electronic components to conventional boards often become broken after only short periods of use.
It would be highly desirable, therefor, to provide a composite printed circuit board having a coefficient of heat expansion closely comparable to the coefficient of heat expansion of the conventional electronic components mounted thereon. In this way, thermal stress between the composite printed circuit board and the electronic components can be reduced.
A printed circuit has been proposed (German Pat. No. 30 35 749) consisting of an iron core and having an anodized aluminum coating as an insulating layer. An intermediate copper layer can be deposited on the iron core according to the patent. The deposition of copper layers on anodized aluminum using electroplating methods is disclosed in U.S. Pat. No. 3,340,164.
This known technique produces layers with relatively good thermal conductivity. However, the anodized aluminum oxide layer produced by electroplating is not entirely satisfactory and, in addition, small cavities, such as boreholes in the composite printed circuit board, are poorly coated using this method.
Accordingly, it would be highly desirable to provide a product and process which permits the low-cost mass production of high-grade printed circuits having high heat dissipation and good insulating properties. It would also be desirable to provide a composite printed circuit having a refractory metal (e.g., molybdenum) core and an inorganic insulating layer, such as aluminum oxide or aluminum nitride. (Aluminum oxide has about three times the heat conductivity of enamel, and about fifteen times the heat conductivity of typical plastics). Moreover, the thermal coefficient of expansion between aluminum oxide and molybdnum are quite compatible and are close to that of silicon (typically used in printed circuit boards). In contrast, the thermal coefficient of expansion of steel, copper and plastics are typically quite different from silicon, thus resulting in thermal stresses.
While there would be considerable value in using aluminum oxide or aluminum nitride as an insulating layer, no effective technique has been suggested for applying these materials onto a refractory metal core for use in composite boards. Nor has a technique been suggested which would apply the aluminum oxide or aluminum nitride layer of sufficient density to reduce the thickness of that layer while providing the desired insulating ability.
Accordingly, it would be highly desirable to provide an aluminum oxide layer in a composite circuit board having good adhesion to the metal core and which is densely composited onto the metal.